1. Field of the Invention
The present invention relates generally to the field of seismic exploration, and more particularly to a seismic vibrator utilizing a hydraulic actuator with a replaceable piston bore liner.
2. Discussion of the Related Art
Typically, seismic exploration involves imparting shock waves, or vibrations, into geologic formations and monitoring the reflected portion of shock waves passing through differing strata in the formations. The reflected shock waves are used to estimate the depth, shape, and composition of differing strata within a formation. Hydraulic actuators are used extensively to generate the shock waves, or vibrations, used in seismic exploration.
Because the world's known supply of hydrocarbon-based fuel continues to dwindle, seismic exploration is pursued in increasingly remote areas thought to contain untapped hydrocarbon deposits. The equipment used to conduct seismic exploration in these remote areas must be dependable and versatile. Further, any problems or breakages experienced by this equipment should be serviceable in the field. Unfortunately, present day seismic vibrators suffer from limited versatility and limited field serviceability. The present invention increases the versatility and field-serviceability of seismic vibrators.
Seismic energy (or shock waves) necessary for seismic exploration is commonly generated in a controlled sweep of frequencies--typically varying from around 10 Hz to 120 Hz. Four to six seismic vibrators may be used for two-dimensional surveys and eight to twelve seismic vibrators may be used for three-dimensional surveys. Seismic energy is imparted to the formation through a base plate attached to the lower end of a vertically disposed, dual acting hydraulic actuator. The force creating the seismic shock wave is generated by hydraulic pressure reacting against the actuator piston and a substantial mass (or mass block). The mass block typically weighs from six to eight thousand pounds and most often forms the body of the hydraulic actuator.
The rigors of seismic exploration often result in mechanical failures. For example, piston rings are known to fail. If a piston ring breaks in a present day seismic vibrator, the piston bore wall is often irreparably damaged. Some prior art hydraulic actuators used as vibrators contain piston bore liners; others do not. If the vibrator has no piston bore liner, the seismic vibrator must be removed from the exploration field to a facility capable of machining the damaged area from the mass block bore. Furthermore, the mass block bore must typically be further expanded, by mechanical removal of mass block material, to accommodate installation of a permanent piston bore liner.
Alternatively, if the vibrator has a conventional piston bore liner, the vibrator must still be removed from the field to a facility capable of machining the damaged piston bore liner from the mass block. Prior art piston bore liners are pressed into the mass block bore as either a shrink fit or interference fit. Either method results in "permanent" engagement of the bore liner with the mass block. In sum, damage to the piston bore in a prior art seismic vibrator requires transporting the vibrator from the exploration site for repair. Transportation of damaged vibrators from exploration sites increases repair costs and delay.
Furthermore, conventional seismic vibrators can typically only generate a fixed peak force. The nominal force imparted to a formation by an actuator is a function of hydraulic pressure and piston surface area. Conventional seismic vibrators operate at constant fluid pressure--typically 3,000 p.s.i. Variation from this constant pressure to change the generated force is not readily achieved, nor accepted, by those in the seismic exploration field. The amount of force generated can also be changed by varying the surface area of the piston. However, this solution requires an inventory of conventional seismic vibrators of varying piston size.
Also, during a typical frequency sweep, many conventional seismic vibrators experience a problem with the inherent spring or bulk modulus of elasticity of the hydraulic fluid used to drive the piston. At high frequency, this fluid spring often creates undesirable harmonics within the vibrator. The undesirable harmonics can be reduced or eliminated by reducing the volume of fluid driving the actuator piston. U.S. Pat. No. 4,785,431 discloses reducing this volume by mechanically increasing the distance between the opposing faces of the piston. U.S. Pat. No. 4,785,431 discloses that others have reduced fluid volume by moving end sleeves or cylinder heads.
The present invention provides a seismic vibrator that overcomes these and other problems found in conventional seismic exploration vibrators.